1. Field of the Invention
This invention relates generally to the field of error correction in bandwidth limited communications systems. More particularly, the invention provides a Frequency Mapping Coding (FMC) scheme for varying the application of error correction redundancy to the transmitted data based on the channel transmission characteristics.
2. Description of Related Art
For bandwidth limited communications environments, such as digital subscriber lines (DSL), characteristics such as the signal to noise ratio (SNR) are not uniform over the useable bandwidth for communication and are typically not even symmetrical. The SNR in the low frequency range, about 1 MHz or below, is much better than in the higher frequency range from 1 MHz to 10 MHz. Traditional quadrature amplitude modulation (QAM) communication systems are designed to work at a symmetric and relatively flat SNR characteristics and, therefore, cannot fully make use of the SNR characteristics over the entire bandwidth. Selection of a QAM scheme is therefore typically limited by the minimum SNR in the available spectrum and those portions of the spectrum having worse SNR characteristics cannot be fully used.
A common alternative solution to overcome the varying SNR over the spectrum is the use of digital multi tone modulation (DMT). In this solution, the modulated signal is divided into different tones spread over the spectrum and a different number of bits is assigned to each tone. The SNR characteristics in the different portions of the spectrum are better utilized. However, the complexity and cost associated with DMT schemes, both to implement and manage, can be significantly higher than QAM approaches. A DSL communication system employing DMT has much higher implementation complexity than a DSL with QAM. Additionally, the spectrum is not really fully used in DMT schemes, since some guard band is needed to separate the adjacent tones.
It is therefore desirable to employ QAM to avoid the complexity associated with DMT, however, correct for the channel characteristics where SNR may impact signal fidelity.
A Frequency Mapped Coding (FMC) scheme is employed to vary the error correction redundancy provided in the communications signal based on the channel characteristics for IQ based modulation. Additional redundancy is added to the coding of the signal in portions of the spectrum where SNR is low and reduced redundancy in the high SNR portions of the spectrum. The matching of the channel spectral characteristics by the FMC combined with the xe2x80x9canalogxe2x80x9d nature of an exemplary QAM modulation, more smoothly fits the available spectrum for better use of the channel capacity. The increased redundancy coding reduces the theoretical bit rate of the QAM channel however the constant bit error rate (BER) is maintained. Trajectories or differential positions rather than the positions of the signal in the constellation are used to measure the degree of vulnerability and add redundancy. The more vulnerable the trajectory is, the more redundancy is added, i.e. less information is transmitted.